Probe for testing a device under test

ABSTRACT

A probe measurement system for measuring the electrical characteristics of integrated circuits or other microelectronic devices at high frequencies.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 11/906,055, filed Sep.27, 2007, which is a continuation of U.S. patent application Ser. No.11/607,398, filed Dec. 1, 2006, which is a continuation of U.S. patentapplication Ser. No. 10/848,777, filed May 18, 2004, now U.S. Pat. No.7,161,363, which is a continuation of U.S. patent application Ser. No.10/445,099, filed May 23, 2003, now U.S. Pat. No. 6,815,963, whichclaims the benefit of U.S. Provisional App. No. 60/383,017, filed May23, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to probe measurement systems for measuringthe electrical characteristics of integrated circuits or othermicroelectronic devices at high frequencies.

There are many types of probing assemblies that have been developed forthe measurement of integrated circuits and other forms ofmicroelectronic devices. One representative type of assembly uses acircuit card on which the upper side are formed elongate conductivetraces that serve as signal and ground lines. A central opening isformed in the card, and a needle-like probe tip is attached to the endof each signal trace adjacent the opening so that a radially extendingarray of downwardly converging needle-like tips is presented by theassembly for selective connection with the closely spaced pads of themicroelectronic device being tested. A probe assembly of this type isshown, for example, in Harmon U.S. Pat. No. 3,445,770. This type ofprobing assembly, however, is unsuitable for use at higher frequencies,including microwave frequencies in the gigahertz range, because at suchfrequencies the needle-like tips act as inductive elements and becausethere are no adjoining elements present to suitably counteract thisinductance with a capacitive effect in a manner that would create abroadband characteristic of more or less resistive effect. Accordingly,a probing assembly of the type just described is unsuitable for use atmicrowave frequencies due to the high levels of signal reflection andsubstantial inductive losses that occur at the needle-like probe tips.

In order to obtain device measurements at somewhat higher frequenciesthan are possible with the basic probe card system described above,various related probing systems have been developed. Such systems areshown, for example, in Evans U.S. Pat. No. 3,849,728; Kikuchi JapanesePublication No. 1-209,380; Sang et al. U.S. Pat. No. 4,749,942; Lao etal. U.S. Pat. No. 4,593,243; and Shahriary U.S. Pat. No. 4,727,319. Yetanother related system is shown in Kawanabe Japanese Publication No.60-223,138 which describes a probe assembly having needle-like tipswhere the tips extend from a coaxial cable-like structure instead of aprobe card. A common feature of each of these systems is that the lengthof the isolated portion of each needle-like probe tip is limited to theregion immediately surrounding the device-under-test in order tominimize the region of discontinuity and the amount of inductive loss.However, this approach has resulted in only limited improvement inhigher frequency performance due to various practical limitations in theconstruction of these types of probes. In Lao et al., for example, thelength of each needle-like tip is minimized by using a wide conductiveblade to span the distance between each tip and the supporting probecard, and these blades, in turn, are designed to be arranged relative toeach other so as to form transmission line structures of stripline type.As a practical matter, however, it is difficult to join the thinvertical edge of each blade to the corresponding trace on the card whilemaintaining precisely the appropriate amount of face-to-face spacingbetween the blades and precisely the correct pitch between the ends ofthe needle-like probe tips.

One type of probing assembly that is capable of providing acontrolled-impedance low-loss path between its input terminal and theprobe tips is shown in Lockwood et al. U.S. Pat. No. 4,697,143. InLockwood et al., a ground-signal-ground arrangement of strip-likeconductive traces is formed on the underside of an alumina substrate soas to form a coplanar transmission line on the substrate. At one end,each associated pair of ground traces and the corresponding interposedsignal trace are connected to the outer conductor and the centerconductor, respectively, of a coaxial cable connector. At the other endof these traces, areas of wear-resistant conductive material areprovided in order to reliably establish electrical connection with therespective pads of the device to be tested. Layers of ferrite-containingmicrowave absorbing material are mounted about the substrate to absorbspurious microwave energy over a major portion of the length of eachground-signal-ground trace pattern. In accordance with this type ofconstruction, a controlled high-frequency impedance (e.g., 50 ohms) canbe presented at the probe tips to the device under test, and broadbandsignals that are within the range, for example, of DC to 18 gigahertzcan travel with little loss from one end of the probe assembly toanother along the coplanar transmission line formed by eachground-signal-ground trace pattern. The probing assembly shown inLockwood et al. fails to provide satisfactory electrical performance athigher microwave frequencies and there is a need in microwave probingtechnology for compliance to adjust for uneven probing pads.

To achieve improved spatial conformance between the tip conductors of aprobe and an array of non-planar device pads or surfaces, severalhigh-frequency probing assemblies have been developed. Such assembliesare described, for example, in Drake et al. U.S. Pat. No. 4,894,612;Coberly et al. U.S. Pat. No. 4,116,523; and Boll et al. U.S. Pat. No.4,871,964. The Drake et al. probing assembly includes a substrate on theunderside of which are formed a plurality of conductive traces whichcollectively form a coplanar transmission line. However, in oneembodiment shown in Drake et al., the tip end of the substrate isnotched so that each trace extends to the end of a separate tooth andthe substrate is made of moderately flexible nonceramic material. Themoderately flexible substrate permits, at least to a limited extent,independent flexure of each tooth relative to the other teeth so as toenable spatial conformance of the trace ends to slightly non-planarcontact surfaces on a device-under-test. However, the Drake et al.probing assembly has insufficient performance at high frequencies.

With respect to the probing assembly shown in Boll et al., as citedabove, the ground conductors comprise a pair of leaf-spring members therear portions of which are received into diametrically opposite slotsformed on the end of a miniature coaxial cable for electrical connectionwith the cylindrical outer conductor of that cable. The center conductorof the cable is extended beyond the end of the cable (i.e., as definedby the ends of the outer conductor and the inner dielectric) and isgradually tapered to form a pin-like member having a rounded point. Inaccordance with this construction, the pin-like extension of the centerconductor is disposed in spaced apart generally centered positionbetween the respective forward portions of the leaf-spring members andthereby forms, in combination with these leaf-spring members, a roughapproximation to a ground-signal-ground coplanar transmission linestructure. The advantage of this particular construction is that thepin-like extension of the cable's center conductor and the respectiveforward portions of the leaf-spring members are each movableindependently of each other so that the ends of these respective membersare able to establish spatially conforming contact with any non-planarcontact areas on a device being tested. On the other hand, thetransverse-spacing between the pin-like member and the respectiveleaf-spring members will vary depending on how forcefully the ends ofthese members are urged against the contact pads of thedevice-under-test. In other words, the transmission characteristic ofthis probing structure, which is dependent on the spacing between therespective tip members, will vary in an ill-defined manner during eachprobing cycle, especially at high microwave frequencies.

Burr et al., U.S. Pat. No. 5,565,788, disclose a microwave probe thatincludes a supporting section of a coaxial cable including an innerconductor coaxially surrounded by an outer conductor. A tip section ofthe microwave probe includes a central signal conductor and one or moreground conductors generally arranged normally in parallel relationshipto each other along a common plane with the central signal conductor soas to form a controlled impedance structure. The signal conductor iselectrically connected to the inner conductor and the ground conductorsare electrically connected to the outer conductor, as shown in FIG. 1. Ashield member is interconnected to the ground conductors and covers atleast a portion of the signal conductor on the bottom side of the tipsection. The shield member is tapered toward the tips with an openingfor the tips of the conductive fingers. The signal conductor and theground conductors each have an end portion extending beyond the shieldmember and the end portions are able to resiliently flex, despite thepresence of the shielding member, relative to each other and away fromtheir common plane so as to permit probing devices having non-planarsurfaces.

In another embodiment, Burr et al. disclose a microwave probe thatincludes a supporting section of a coaxial cable including an innerconductor coaxially surrounded by an outer conductor, as shown in FIGS.2A, 2B, and 2C. A tip section of the microwave probe includes a signalline extending along the top side of a dielectric substrate connecting aprobe finger with the inner conductor. A metallic shield may be affixedto the underside of the dielectric substrate and is electrically coupledto the outer metallic conductor. Ground-connected fingers are placedadjacent the signal line conductors and are connected to the metallicshield by way of vias through the dielectric substrate. The signalconductor is electrically connected to the inner conductor and theground plane is electrically connected to the outer conductor. Thesignal conductor and the ground conductor fingers (connected to theshield via vias) each have an end portion extending beyond the shieldmember and the end portions are able to resiliently flex, despite thepresence of the shielding member, relative to each other and away fromtheir common plane so as to permit probing devices having non-planarsurfaces. While the structures disclosed by Burr et al. are intended toprovide uniform results of a wide frequency range, they unfortunatelytend to have non-uniform response characteristics at high microwavefrequencies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an existing probe.

FIGS. 2A-2C illustrate an existing probe.

FIG. 3 illustrates a modified probe.

FIG. 4 illustrates a side view of a portion of the probe of FIG. 3.

FIG. 5 illustrate a bottom view of a portion of the probe of FIG. 3.

FIG. 6 illustrates a force versus vertical probe deformation graph.

FIG. 7 illustrates probe pre-loading.

FIG. 8 illustrates a force versus vertical probe deformation graph forprobe pre-loading.

FIG. 9 illustrates a probe contact.

FIG. 10 illustrates a modified probe contact.

FIG. 11 illustrates contact resistance.

FIG. 12 illustrates contact resistance.

FIG. 13 illustrates a membrane type probe tip.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The present inventors considered the co-planar fingered probing devicesdisclosed by Burr et al., including the co-planar finger configurationand the microstrip configuration with fingers extending therefrom. Inboth cases, electromagnetic fields are created during probing betweenthe fingers. These electromagnetic fields encircle each of the fingers,electrically couple the signal finger to the ground fingers, andelectrically couple the ground fingers one another. While the probingdevice is being used for probing, the resulting electromagnetic fieldssurrounding the fingers interact with the wafer environment. Whileprobing in different regions of the wafer, the interaction between theelectromagnetic fields around the fingers and the wafer change,typically in an unknown manner. With a significant unknown change in theinteraction it is difficult, if not impossible, to accurately calibrateout the environmental conditions while probing a device under test.

When multiple probes are being simultaneously used for probing the samearea of the wafer, the probe tips come into close proximity with oneanother and result in additional coupling between the probes, normallyreferred to as cross-talk. In addition, the region between the supportfor the fingers, such as a dielectric substrate, and the extendedportion of the fingers results in a significant capacitance, whichimpedes high frequency measurements.

The present inventors were surprised to determine that the microstripstructure disclosed by Burr et al. further does not calibrate well oncalibration test substrates at very high frequencies, such as in excessof 70 GHz. This calibration is independent of potential interaction witha wafer at a later time during actual probing of a device under test.After examination of this unexpected non-calibration effect the presentinventors speculate that an energy is created in an “undesired mode”,other than the dominant field modes, at such extreme frequencies. This“undesired mode” results in unexpected current leakages from the signalpath thus degrading the signal integrity. The present inventors furtherspeculate that this “undesired mode” involves resonating energy in theground plane as a result of discontinuities in the ground path,including for example, the connection between the ground plane and theexternal portion of the cable, and the inductance in the ground plane.This ground plane resonant energy results in unpredictable changing ofthe energy in the signal path to the device under test, thus degradingperformance. This degradation wasn't apparent at lower operatingfrequencies, so accordingly, there was no motivation to modify existingprobe designs in order to eliminate or otherwise reduce its effects.

Referring to FIG. 3, a semi-rigid coaxial cable 40 is electricallyconnected at its rearward end to a connector (not shown). The coaxialcable 40 normally includes an inner conductor 41, a dielectric material42, and an outer conductor 43. The coaxial cable 40 may likewise includeother layers of materials, as desired. The forward end of the cable 40preferably remains freely suspended and, in this condition, serves as amovable support for the probing end of the probe.

A microstrip style probe tip 80 includes a dielectric substrate 88 thatis affixed to the end of the coaxial cable 40. The underside of thecable 40 is cut away to form a shelf 85, and the dielectric substrate 88is affixed to the shelf 85. Alternatively, the dielectric substrate 88may be supported by an upwardly facing shelf cut away from the cable orthe end of the cable without a shelf. Referring also to FIG. 4, aconductive shield 90, which is preferably planar in nature, is affixedto the bottom of the substrate 88. The conductive shield 90, may be forexample, a thin conductive material (or otherwise) that is affixed tothe substrate 88. By using a generally planar conductive material havinga low profile the shield 90 is less likely to interfere with the abilityto effectively probe a device under test by accidently contacting thedevice under test. The conductive shield 90 is electrically coupled tothe outer conductor 43 to form a ground plane. The outer conductor 43 istypically connected to ground, though the outer conductor 43 may beprovided with any suitable voltage potential (either DC or AC). Theconductive shield 90 preferably covers all of the lower surface of thesubstrate 88. Alternatively, the conductive shield 90 may cover greaterthan 50%, 60%, 70%, 80%, 90%, and/or the region directly under amajority (or more) of the length of a conductive signal trace on theopposing side of the substrate 88.

One or more conductive signal traces 92 are supported by the uppersurface of the substrate 88. The conductive traces 92, may be forexample, deposited using any technique or otherwise supported by theupper surface of the substrate. The conductive trace(s) 92 iselectrically interconnected to the inner conductor 41 of the coaxialcable 40. The inner conductor 41 of the coaxial cable 40 and theconductive trace(s) 92 normally carries the signal to and from thedevice under test. The conductive trace(s) 92 together with the shieldlayer 90 separated by a dielectric material 88 form one type of amicrostrip transmission structure. Other layers above, below, and/orbetween the shield 90 and the conductive trace 92 may be included, ifdesired.

To reduce the effects of the aforementioned unexpected high frequencysignal degradation, the present inventors determined that the signalpath may include a conductive via 94 passing through the substrate 88.The conductive via 94 provides a manner of transferring the signal pathfrom the upper surface of the substrate to the lower surface of thesubstrate. The conductive via 94 avoids the need for using a conductivefinger extending out from the end of the substrate 88 that wouldotherwise result in a significant capacitance between the extendedfinger and the end of the substrate 88. The conductive via 94 provides apath from one side of the substrate 88 to the other side of thesubstrate 88 in a manner free from an air gap between the conductive via94 and the substrate 84 for at least a majority of the thickness of thesubstrate 88. In addition, the shield layer 90 preferably extends beyondthe via 94 to provide additional shielding.

Referring also to FIG. 5, the lower surface of the substrate 88illustrates a contact bump 100 electrically connected to the via 94 andthe trace 92 extending below the lower surface of the substrate 88 andthe shield 90 which may be used to make contact with the device undertest during probing. The conductive shield 90 may include an “patterned”section around the contact “bump” 100 so that the shield and the signalpath are free from being electrically interconnected (e.g., the shieldlayer 90 may be greater than 50%, 75%, or laterally surrounding all ofthe contact at some point). It is to be understood that the contact maytake any suitable form, such as a bump, a patterned structure, or anelongate conductor. The conductive shield 90 may laterally encircle theconductive bump which increases the resistance to externalelectromagnetic fields. Also, the conductive shield 90 extending beyondthe conductive bump 100 reduces the crosstalk from other probes. Forsome probing applications, one or more shield contacts 102 may beprovided, if desired. The shield layer and the conductive trace arenormally constructed to provide a microstrip transmission linecontrolled impedance structure.

To further increase the performance at high frequencies the presentinventors considered the effects of the substrate material. In manycases the dielectric constant of the dielectric substrate material ishigh, such as Al.sub.2O.sub.3 which has a 9.9 dielectric constant.Materials with a high dielectric constant have a tendency to concentratethe electromagnetic fields therein, thus decreasing the electromagneticfields susceptible to influence by other devices. In addition, thethickness of the substrate is typically 250-500 microns to providemechanical stability. Thus the fields tend to concentrate within thesubstrate.

Referring to FIG. 13, while considering such substrates the presentinventors came to the realization that the flexible membrane substratemay be substituted for the more rigid substrate 88. An example ofmembrane material is described in U.S. Pat. No. 5,914,613. In general,membrane based probes are characterized by a flexible (or semi-flexible)substrate with traces supported thereon together with contactingportions being supported thereon. The contacting portions come intocontact with the device under test and the traces are normally on theopposing side of the membrane connected to the contacting portions usingvias. In many cases, the membrane technology may be significantlythinner than ceramic based substrates, (see e.g., substrate 88) such as40, 30, 20, 10, 5, or 3 microns or less. Normally the dielectricconstant of the membrane material is 7 or less, sometimes less than 6,5, or 4 depending on the particular material used. While normally usinga lower dielectric constant substrate is unsuitable, using asignificantly thinner substrate together with a lower dielectricconstant substrate raises the theoretical frequency range of effectivesignal transmission to 100's of GHz. The significantly thinner substratematerial permits positioning the lower shield material significantlycloser to the signal traces than the relatively thick ceramic substrate,and therefore tends to more tightly confine the electromagnetic fieldsthere between.

When the membrane based probe comes into contact with a device undertest, as in most probes, it tends to skate across the pad as additionalpressure is exerted. This skating is the result of the angled probeand/or co-axial cable flexing while under increasing pressure againstthe test pad. A limited amount of skating is useful to “scrub” awayoxide layers, or otherwise, that may be built up on the contact pad,which results at least in part from a suitable amount of pressure and/orskating. In many cases the test pad is typically relatively small andexcessive skating from slightly too much pressure being applied resultsin the probe simply skating off the test pad. In addition, if excessivepressure is exerted damage to the probe and/or contact pad may result.Accordingly, there is an acceptable range of pressure and skating thatshould be maintained.

Referring to FIG. 6, for purposes of illustration the force applied bythe probe versus vertical deformation of the probe as a result of theforce being applied is shown. Line 400 is for a low stiffness probe andline 402 is for a high stiffness probe. Vertical line 404 illustratesthe maximum skating distance before the probe is likely off the contactpad, and accordingly the greatest distance of over travel of the probeafter contact with the contact pad. Vertical line 406 illustrates theminimum generally acceptable skating distance of the probe to ensuresufficient scrubbing distance of the oxide layer or otherwise that maybe built up on the contact pad, and accordingly the minimum generallyacceptable distance of over travel of the probe after contact with thecontact pad. Typically the range of useful over-travel is approximately50 to 200 microns. Horizontal line 408 illustrates the maximumacceptable force that the probe may apply so that damage to the probeand/or contact pad is minimized. Horizontal line 410 illustrates theminimum acceptable force that the probe should apply so that sufficientpressure is exerted to break through the oxide layer or otherwise thatmay be built up on the contact pad.

It may be observed that there is a rectangular region (in this example)within which acceptable over-travel and force is applied by the probe tothe contact pad. For the low stiffness probe 400 a range of 420 is shownwhere acceptable probing takes place. It may be observed that thisdistance uses less than the maximum range between vertical lines 404 and406, and thus the over-travel must be carefully controlled by theoperator. For the high stiffness probe 402 a range of 422 is shown whereacceptable probing takes place. It may be observed that this distanceuses less than the maximum range between vertical lines 404 and 406, andthus the over-travel must be carefully controlled by the operator.Accordingly, the stiffness of the probe needs to be carefullycontrolled, which is difficult, in order to establish an acceptableoperating region. Further, it is noted that there is some relationshipbetween skate to over-travel which may be controlled also. To littleskate is problematic because some scrubbing action improved contactresistance and lateral motion provides visual confirmation (through themicroscope) that contact has been made. To much skate is problematicbecause then the probe tip slides across and off the pad before gettingenough force for good contact resistance. The pre-load provides theopportunity to tune that ratio by varying the curvatures of the probeand the pre-load location.

After consideration of the limitations seemingly inherent with thestiffness of the probe, the present inventors came to the realizationthat by using a relatively flexible probe together with pre-loading aportion of the force to be applied by that probe, a modifiedforce-distance profile may be obtained that is more readily within theacceptable region. The modified force-distance profile may include moreof the acceptable probing range, namely a wider probing range within therectangular region, than otherwise achieved if the probe were nototherwise modified. Referring to FIG. 7, this pre-loading may beachieved by using a string 440 or other support member to upwardly flexthe probe. If the low stiffness probe 400 is used, then a modified forceprofile 444 (see FIG. 8) may be obtained. It is noted that the lowercurved portion 446 is as a result of the pre-loading of the probe. Theupper portion 448 is a result of the probe itself and generally has thesame force slope as the probe without pre-loading. It may be observedthat in this manner a probe profile that has a relatively low slope thatis suitable to extend across more of the useful probing range whilemaintaining reasonable forces may be used. The pre-loading raises theinitial force to a range near the minimum generally acceptable force.The height of the profile 444 may be modified by adjusting thepre-loading. Also, the slope of the profile 444 may be lessened byselecting a more flexible probe or otherwise modifying the orientationof the probe in relation to the contact pad. This pre-load system, whileespecially useful for membrane type probes, is likewise useful withother probing technologies.

When making probing measurements the contact resistance between theprobe and the device under test is an important consideration. The tipof the probe may be designed in such a manner as to achieve a lowcontact resistance while permitting effective viewing of the area to beprobed with an associated microscope. The probe tip 438 (see FIG. 9) istypically constructed in such a manner that the resulting structure hasa pair of opposing inclined surfaces 450 and 452. The tip 454 of theprobe is preferably extended from the inclined surfaces 450 and 452. Theconstruction of the probe tip may be done using a sacrificial substrateinto which is created a depression. Into this depression is locatedconductive material, traces are located thereon if desired, and flexibledielectric material is located on or under the traces. See U.S. Pat. No.5,914,613, incorporated herein by reference, together with allreferences cited herein. Thereafter the sacrificial substrate is removedleaving the probe tip, traces, and membrane material. The probe tip 438is acceptable, however, it is difficult to see the region proximate thetip 438 when contacting the device under test because of the inclinedsurfaces 450 and 452.

To improve the visibility of the tip 438 during probe it has beendetermined that the probe 454 may be ground back or otherwise a portionof the probe removed, as illustrated in FIG. 10. By removal of a portionof the probe a greater visibility may be achieved during probing of thedevice under test as illustrated in FIG. 10. It is also to be understoodthat the probe may be constructed in a manner such that a portion of theprobe does not need to be removed. The tip portion 454 is preferablyapproximately 12 .mu.m.times.12 .mu.m, with about 2-3 mills of verticalover-travel resulting in about 1 mil of longitudinal tip scrub. Theprobe may likewise retain a lip 460 that provides additional structuralsupport for the tip 454. The backside 462 of the probe may even beundercut with respect to the plane of the base 464 of the probe.Alternatively, the backside 462 of the probe may be within 30 degrees ofvertical (undercut or not) with respect to the plane of the base 464 ofthe probe.

The contact resistance resulting from the described structure turns outto be exceedingly low, especially in comparison to other types ofprobing systems like Tungsten Probes. Referring to FIG. 1, the contactresistance on un-patterned aluminum is less then 30 m.OMEGA. over 5000contact cycles, which is considerably lower than conventional tungstenprobes where the contact resistance is approximately 130 m.OMEGA.Referring to FIG. 12, with the probe being held in contact with thealuminum pads the contact resistance is shown as a function of time. Asillustrated in FIG. 12, only 10 m.OMEGA. of variation was observedduring a 5-hour interval. In a similar test, conventional tungstenprobes show significant changes over the same period, typically thecontact resistance varies from 35 m.OMEGA. to 115 m.OMEGA.

Another consideration in the design of the probe is the characteristicsof the different transmission structures. The coaxial cables providegood high frequency transmission characteristics. Within the membranestructure, connected to the coaxial cables, the micro-strip structureprovides good high frequency characteristics. Connected to the membranestructure includes a set of contacts, such as a signal contact and apair of ground contacts. The contacts provide a co-planar transmissionstructure which has less bandwidth capability than the coaxial cable andthe micro-strip structure. To achieve acceptable bandwidth the presentinventors have determined that the contacts should be no longer than 150microns. More preferably the contacts should be no longer (e.g., heightfrom planar surface) than 100 microns, or no longer than 75 microns, orno longer than 55 microns.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A probe comprising: (a) a dielectric substrate supporting an elongateconductor on said substrate and a conductive member supported on saidsubstrate; (b) a contact electrically interconnected to said conductivepath for testing a device under test; (c) wherein said elongateconductor and said conductive member form a controlled impedancestructure; (d) wherein said probe has a characteristic that its contactresistance on un-patterned aluminum is generally less than 30 mΩ over5000 contact cycles.
 2. The probe of claim 1 further comprising aflexible structure interconnected with said dielectric substrate, and apre-loading mechanism that pre-loads said flexible structure with aforce when said contact is free from being engaged with said deviceunder test in such a manner that when force is applied to said probe ithas a non-linear vertical deformation profile versus force applied. 3.The probe of claim 1 further comprising a conductive path between saidfirst side of said substrate and said second side of said substrate. 4.The probe of claim 1 wherein said controlled impedance structure is amicrostrip.
 5. The probe of claim 2 wherein said flexible structure is aco-axial cable.
 6. The probe of claim 1 herein said substrate has athickness of less than 40 microns with a dielectric constant of lessthan
 7. 7. The probe of claim 3 wherein said conductive path is in aregion within the periphery of said substrate for at least a majority ofthe thickness of said substrate.
 8. The probe of claim 1 wherein anelongate conductor is electrically interconnected to a central conductorof a coaxial cable.
 9. The probe of claim 1 wherein said conductivemember is electrically connected to a conductor surrounding said centralconductor of said coaxial cable.
 10. The probe of claim 1 wherein saidcontact resistance is generally less than 5 mΩ over said 5000 cycles.11. A probe comprising: (a) a dielectric substrate supporting anelongate conductor on said substrate and a conductive member supportedon said substrate; (b) a contact electrically interconnected to saidconductive path for testing a device under test; (c) wherein saidelongate conductor and said conductive member form a controlledimpedance structure; (d) wherein said probe has a characteristic thatits contact resistance on un-patterned aluminum is generally less than35 mΩ over a 5 hour time period.
 12. The probe of claim 11 wherein saidcontact resistance is generally less than 10 mΩ over said 5 hour timeperiod.
 13. The probe of claim 11 wherein said contact resistance hasgenerally less than 10 mΩ of variation over said 5 hour time period. 14.The probe of claim 11 further comprising a flexible structureinterconnected with said dielectric substrate, and a pre-loadingmechanism that pre-loads said flexible structure with a force when saidcontact is free from being engaged with said device under test in such amanner that when force is applied to said probe it has a non-linearvertical deformation profile versus force applied.
 15. The probe ofclaim 11 further comprising a conductive path between said first side ofsaid substrate and said second side of said substrate.
 16. The probe ofclaim 11 wherein said controlled impedance structure is a microstrip.17. The probe of claim 14 wherein said flexible structure is a co-axialcable.
 18. The probe of claim 11 herein said substrate has a thicknessof less than 40 microns with a dielectric constant of less than
 7. 19.The probe of claim 15 wherein said conductive path is in a region withinthe periphery of said substrate for at least a majority of the thicknessof said substrate.
 20. The probe of claim 11 wherein an elongateconductor is electrically interconnected to a central conductor of acoaxial cable.
 21. The probe of claim 11 wherein said conductive memberis electrically connected to a conductor surrounding said centralconductor of said coaxial cable.